1. Field of the Invention
The present invention generally relates to emissions control for small four-cycle gasoline engines and, more particularly, to an improved carburation device which greatly reduces emissions while maintaining acceptable levels of performance.
2. Description of the Prior Art
Emissions from gasoline engines is a major air pollution problem. California has been a leader in enacting regulations aimed at reducing gasoline engine emissions. Until very recently, most of these regulations have been targeted towards automobile emissions. However, small gasoline engines commonly used to power lawn-mowers, emergency generators, and the like, have now been recognized as significant contributors of harmful emissions. Accordingly, California has passed emissions regulations which are estimated to effect about 500,000 small four-cycle engines by the beginning of 1994. Other states as well as the EPA nationally are expected to follow suit in due course.
Typical automobile emission reduction systems include catalytic converters and fuel control systems. In recent years, computer controlled fuel injection systems have largely replaced conventional carburetors in most automobiles. Unfortunately, the price and complexity of such systems make them impractical for small engine adaptation.
When a cold engine is first started, a richer mixture is required to insure stable combustion until fuel vaporization improves. Simplified temperature driven automatic chokes, similar to the type once used in automobiles, have been tried on small engines. However, these implementations have been unsuccessful due to the difficulty of providing a uniformly rich mixture over a wide range of operating conditions. For example, a simple automatic choke (or manual choke) will tend to flood the engine with excess fuel if the throttle is opened too wide for the current choke setting. In order to avoid these problems, many small engines today use a choke or primer to briefly supply extra fuel for easy starting and tune the carburetor fuel/air mixture rich. Unfortunately, the carburetor remains tuned rich after warm-up and continues to supply excess fuel throughout the operation and subsequently results in very high emissions. Additionally, when the throttle is closed suddenly, the fuel supply is momentarily too great. This excess fuel causes a large excursion in carbon monoxide (CO) and hydrocarbon (HC) emissions, and sometimes cause rich misfire.
In order to reduce HC and CO emissions a leaner fuel/air mixture for the current load condition is desired. One way to achieve this result is to use a smaller carburetor jet to lean the mixture. Leaning the mixture in this manner decreases emissions but also introduces several new problems. The choke function must be improved to compensate and allow smooth running during warm-up. Throttle response (or response to an increased load) suffers because the existing carburetors have no acceleration enrichment mechanism other than using a rich mixture at all times. Also, the leaner mixture may not provide the fuel cooling needed under high load conditions to prevent engine overheating.
U.S. Pat. Nos. 3,568,437 to Briggs and 4,483,309 to Norman et al. disclose internal combustion engine carburetors which are designed to reduce emissions by leaning the fuel/air mixture with fresh air using a bypass air inlet with valve mechanically linked to the throttle. As the throttle is opened wider, the bypass valve is also opened wider to vent ambient air to lean the mixture. This reduces emissions, but performance suffers. When the throttle is suddenly opened, such as during acceleration, the bypass valve also opens resulting in a fuel/air mixture which is too lean for suitable acceleration response. Additionally, if the throttle is suddenly closed, the instant depravation of ambient air will cause the mixture to become too rich causing excess emissions and possible misfires.